ath79: add correct phy-mode for QCA9563 boards

ath79: add correct phy-mode for QCA9563 boards

The TP-Link RE450 as well as the UniFi AC series missed the phy-mode
property. Because of this, the incorrect MII phy-mode from the root dtsi
was used.

With Kernel 5.4, this leads to problems when used with a AR8033 PHY. The
bootloader seems to leave the fiber pages selected.

As there's not switch to copper pages happening in at803x_config_init
due to the incorrect phy-mode, the new at803x_read_status will interpret
the status of the SGMII side as the status of the copper side.

Signed-off-by: David Bauer <mail@david-bauer.net>